If i were doing this i would look hard at a boost reg configured as a constant current source. At least two sets. On Oct 19, 2013 9:38 AM, "Carl Denk" wrote: > Yes, I was aware of the cable losses, and where I was thinking LM317 at > the end of cable, I do have 2 of those MPJA regulators per the link that > I have recently got. Thanks again for the input. > On 10/19/13 11:10 AM, Denny Esterline wrote: > > Missing in the discussion is the non-trivial resistance in the cable... > > 24Ga wire is variously reported at about 0.025 ohms per foot. 1000' dow= n > > and 1000' back and you have 50ohms of added resistance to the circuit. > > If I'm reading your description right, 130mA for the camera and 50mA fo= r > > LEDs, 50ohms*180mA, you'll be loosing 9 volts to the cable alone. I'd b= e > > surprised if this works at all. Probably getting by at the moment becau= se > > you're only using 320' of cable and doubling up the pairs - which cuts > your > > cable loss to about 1.5 volts. > > > > I'd recommend increasing voltage and putting a switching regulator on t= he > > end. > > > > I tripped on this one the other day > > > http://www.mpja.com/LM2596-Step-Down-Adjustable-15-37V-DC_DC-Converter/pr= oductinfo/30148%20PS/ > > Probably > > not the "best" depending on your metrics, but rates high in the "easy", > > "cheap for a one-off", "small enough" and "available off the shelf" > > categories. > > > > -Denny > > > > > > > > On Sat, Oct 19, 2013 at 7:33 AM, RussellMc wrote: > > > >> LEDs should (~~=3D must) be driven by constant current. > >> Trying to drive them with ~=3D their notional Vf will lead to a > substantial > >> variation in most cases. > >> When operating on 12V you should have only 3 LEDs in series, plus > current > >> control.. > >> When operating with 24V you can have up to about 6 LEDs in series plus > >> current control. > >> The more "headroom (aka spare voltage) that you have the easier it is = to > >> get ~=3D constant current with simple circuits. > >> > >> It is exceeding bad practice (also exceedingly bad practice) to attemp= t > to > >> drive two parallel strings of LEDs without any attempt to current > balance. > >> I have seen some LEDs that are so well matched for Vf that you could > >> 'almost get away with" doing this, but most LEDs have enough variation= , > >> even in the same batch, to make this problematic. > >> > >> LEDs should be driven at MORE than their rated combined Vf with a > constant > >> current source taking up the difference. You can use as little as an > LM317 > >> and one resistor as a constant current source. > >> You'll need about 3.5V "headroom" for an LM317 to operate as a current > >> source. > >> eg for 4 LEDs. V min ~=3D 3.8V x 4 =3D 15.2V. > >> Headroom =3D 3.5V say. > >> Vinmin=3D 3.5 + 15.2 >=3D 18.7V. > >> ie ~=3D 24V would be dandy. > >> > >> Vin =3D LM317 Vin > >> Iout =3D LM317 ADj. > >> Rcc from Vout to Vadj. > >> Rcc =3D 1.25 / I A =3D 1250 / I_ma > >> Dissipation in R =3D~ 6/R or 1.25 mW per mA > >> eg for 25 mA. > >> R =3D 1.25 / .025 =3D 1250 / 25 =3D 50 Ohms. > >> Pd_Rcc =3D 50 x 1.25 ~=3D 63 mW. > >> > >> If you have 4 LEDs in series and this is unchangeable, you could short > one > >> out per group and probably get a better result. > >> > >> If these are on only occasionally then 'any brand will do", almost. If > they > >> are intended to spend thousands of hours on over their lifetime then y= ou > >> should use LEDs from the major LED manufacturers OR ones made under > licence > >> to them. LED lifetimes are still woefully low in many cases - despite > >> claims to the contrary. > >> > >> > >> Russell > >> -- > >> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > >> View/change your membership options at > >> http://mailman.mit.edu/mailman/listinfo/piclist > >> > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .